A typical hydraulic-set packer 20 as shown in FIG. 1 has a mandrel 22 with a piston 30 and a packing element 40 disposed thereon. The mandrel 22 has a female thread 23a at an uphole end and a male thread 23b at a downhole end for mating to components of a tubing string or the like. When deployed downhole, fluid pumped in the mandrel 22 passes through a port 24 and enters a space 26 adjacent the piston 30. The pumped fluid forces the piston 30 toward the packing element 40, causing the piston 30 to push a lower gage ring 42 against the packing element 40 and sandwich it against an upper gage ring 44. Meanwhile, an outside serrated surface of the moving piston 30 successively engages a ratchet mechanism 35 that prevents movement of the piston 30 away from the packing element 40.
As the piston 30 compresses it, the packing element 40 expands radially outward to the wall 12 of a surrounding casing, borehole, or tubular. The expanded packing element 40 is depicted by dashed lines at 40′. Once set, the packing element 40 isolates the annulus 12 into separate portions 14a and 14b. 
As the packing element 40 is being set, however, fluid can become trapped in the downhole annulus portion 14b, especially if another packer (not shown) is set downhole from the packer 20. For this reason, the piston 30 that sets the packing element 40 typically travels in a direction away from fluid that may become trapped by the packing element 40. In other words and as shown more particularly in FIG. 1, the piston 30 travels uphole toward the packing element 40 away from the downhole annulus portion 14b in which fluid may become trapped as the packing element 40 is set.
Having the piston 30 travel away from potentially trapped fluid is the typical configuration used in the art so the packing element 40 can seal properly. If the piston 30 were instead moved towards potentially trapped fluid, then the packing element 40 may not completely set because incompressible fluid being trapped by the expanding packing element 40 could prevent the packing element 40 from traveling far enough to completely seal with the surrounding wall 12. The result is that the packing element 40 may not produce an adequate seal.
The typical configuration of moving the piston 30 away from trapped fluid can also complicate how such a packer 20 is deployed and used downhole for a given implementation. For example, the portion of the packer 20 having the piston 30 must be of sufficient length to accommodate the required mechanisms to set the packing element 40 in a direction away from trapped fluid. This can directly increase the distance that the packing element 40 can be from other wellbore components used downhole. For example, the increased distance can be disadvantageous in some implementations because a larger expanse of the annulus may need to be isolated than ideally desired.